How to remove nested parentheses in LISP

Question

How can I remove nested parentheses recursively in Common LISP Such as

  (unnest \'(a b c (d e) ((f) g))) => (a b c d e f g)
  (unnest \'(a b))                    


        

Answer 1

Lisp has the function remove to remove things. Here I use a version REMOVE-IF that removes every item for which a predicate is true. I test if the thing is a parenthesis and remove it if true.

If you want to remove parentheses, see this function:

(defun unnest (thing)
  (read-from-string
   (concatenate
    'string
    "("
    (remove-if (lambda (c)
                 (member c '(#\( #\))))
               (princ-to-string thing))
    ")")))

Note, though, as Svante mentions, one does not usually 'remove' parentheses.